The invention is directed to a tire pressure sensor for motor vehicles.
A tire pressure sensor for sensing the pressure in a tire of a motor vehicle is known. This tire pressure sensor comprises a housing fixed on a wheel rim of a vehicle wheel, on which a tire whose pressure is to be measured is mounted, and a pressure switch which is mounted at the circumference of the wheel rim extending in a radial direction into the interior of the tire. The pressure switch includes a reference pressure chamber, which is closed on a side closet the tire interior by an electrically conducting contact diaphragm, which is arranged parallel to the tread of the tire and which respect to the housing, according to the relative tire pressure and is lifted according to speed of the tire by a mass, which is movable in a radial direction by centrifugal force for the purpose of speed-dependent raising of the switching threshold of the pressure switch.
Tire and vehicle manufacturers recommend increased tire pressures for higher vehicle speed. This can be taken into account in that the switching threshold is adjusted to the greatest required pressure by adjusting the pressure in the reference pressure chamber. This results in a loss in comfort at low speeds (compare DE-OS 32 43 845). On the other hand, if the tire pressure sensor is adjusted to the tire pressure recommended in the lower speed range at full load, the required higher tire pressure at high speeds is not monitored and the tire pressure monitoring device gives a false sense of security, which is dangerous precisely in this speed range.
It is known from DE-OS 26 26 475 to provide the diaphragm of the reference pressure chamber of a tire pressure sensor with a solid piston whose front side is contacted by the tube of the tire to be monitored, which presses in the diaphragm against the pressure in the reference pressure chamber, when there is sufficient air pressure in the tire, to the extent that an electrical contact in the reference pressure chamber is closed It is further mentioned in DE-OS 26 26 475 that centrifugal forces at the piston of the diaphragm advantageously reduce the force exerted on the diaphragm by the tire pressure at increasing speed, so that the tire pressure must be increased at higher speeds in order to keep the switching contact closed. However, it is disadvantageous that the piston which is used as added mass for the diaphragm is covered by the tube of the tire, so that the centrifugal forces acting on it act on the reference pressure chamber only in part and in an undefined manner for raising the switching threshold. It is therefore not possible with this solution to raise the switching threshold by a defined and reproducible amount at a determined speed relative to the stationary state, since the speed-dependent raising of the switching threshold can not be determined beforehand in this device.